Many forms of breathing apparatus are known in the art which include a breathing mask. Such breathing masks may be either a full-face mask or a half-face mask. In many cases a microphone is disposed within the breathing mask so that the wearer of the mask can communicate with others either through a radio or a communications amplifier worn by the wearer of the breathing mask. The breathing apparatus may be either a continuous flow system in which there is a constant flow of air into the breathing mask or a demand system where air is introduced into the mask only in response to a system pressure drop. Virtually all self-contained breathing apparatus today are of the demand type. Communication in such apparatus is adversely effected by the introduction of air into the mask in response to a system pressure drop, since typically a hissing noise is made when the air is introduced into the mask, which hissing noise will be picked up by the microphone and communicated unless some means is provided to either decrease or shut off the output of the microphone during inhalation.
A large number of proposals have been made in the past for controlling this problem. In one approach the microphone is mounted at a location where it is not likely to pick up the sound of incoming air. This approach is shown in British Patent 396,904. Another approach has been to mount the microphone within the breathing mask but to use a sound canceling microphone where background sounds impinge upon both sides of the microphone, such microphones being well known in the art. Another approach, as shown in U.S. Pat. No. 4,154,981, is to attenuate the output of the microphone when high noise levels are present.
By far the most common approach which has been used for controlling the output of a microphone has been simply toggling the microphone circuit between "on" and "off" conditions. This can be done manually, but manual actuation of course requires the user of the breathing apparatus to switch the microphone "on" and "off". This switching requires both manual and mental effort. In some circumstances, for example an emergency escape situation, thoughts may be elsewhere and hands may not be free. U.S. Pat. No. 4,382,159 proposes to overcome the disadvantages of manual switching by providing a blow-actuated microphone which switches a microphone between its "on" and "off" states in response to a blowing effort. While this proposal overcomes the disadvantages of requiring a manual switch, it still requires a mental effort on the part of the wearer. The above patent also makes reference to voice-actuated keying devices which switch the mechanism between on and off states, and one such approach is shown in U.S. Pat. No. 4,119,797 which uses a voice signal to switch the communications system between its "on" and " off" states. Typically such devices cannot distinguish between voice and noise and therefore such devices have not met with substantial commercial success. It has also been proposed to add a sensor which senses the flow in the gas line to a breathing mask, such additional equipment being shown in U.S. Pat. Nos. 4,181,835 and 4,799,263. While such approaches require neither mental nor manual effort, they have the disadvantage in that they are add-on devices disposed in the flow line to the breathing mask. These devices cause a pressure drop in the gas line. In addition, failure of the unit could plug the breathing line and render the breathing mask unusable.